In 1974 Herman van den Berghe et al.1 reported a distinct hematologic disorder associated with acquired deletion of the long arm of chromosome 5 [del(5q)]. This novel nosological entity was described in more detail one year later by Sokal, van den Berghe, and coworkers.2 Patients with del(5q) had macrocytic anemia with oval macrocytes, normal to slightly reduced white blood cell counts, and normal to elevated platelet counts. With respect to the bone marrow, there was erythroid hypoplasia but “the most striking abnormality concerned the megakaryocytes and especially their nuclei, which were generally small, round or oval, and nonlobulated”.2 These morphological abnormalities are illustrated in Figure 1. Until that time, the only specific chromosomal abnormality in hematologic disorders was the Philadelphia chromosome associated with chronic myeloid leukemia.3,4 Sokal et al.2 concluded that del(5q) represented a novel specific chromosomal abnormality associated with refractory anemia, although they had no explanation to connect the abnormal chromosome 5 with the hematologic manifestations.
The 5q- syndrome
Subsequent studies showed that a chromosome 5q deletion can be found in different myeloid disorders, and underscored the need to define the 5q- syndrome properly. Boultwood and Wainscoat5 proposed the following simple definition of the 5q- syndrome: primary myelodysplastic syndrome (MDS) with del(5q) as the sole karyotypic abnormality and without excess of blasts. In their experience, patients with the 5q- syndrome so defined had macrocytic anemia, a normal or increased platelet count, hypolobular megakaryocytes, and a low risk of transformation to acute myeloid leukemia. By studying these patients, Boultwood and co-workers6 found that the common deleted region of the 5q- syndrome was the approximately 1.5-megabase interval at 5q31-q32 flanked by D5S413 and the GLRA1 gene. This region is distinct from that of the 5q deletion at 5q31 of malignant myeloid disorders such as acute myeloid leukemia or therapy-related MDS.7,8
In 2001 the World Health Organization (WHO) published a new classification for hematopoietic and lymphoid neoplasms that recognized the MDS with isolated del(5q) – the 5q- syndrome – as a unique, narrowly defined entity.9 According to the WHO classification, additional cytogenetic abnormalities or 5% or more blasts in the blood or marrow excludes the diagnosis of 5q- syndrome. Indeed, a subsequent study showed that within MDS patients with del(5q), those with excess of blasts and those with an additional chromosomal abnormality have a significantly shorter overall survival than patients with isolated del(5q).10 In this issue of the journal, Wang et al.11 report on studies of genome-wide analysis of copy number changes and loss of heterozygosity in patients with MDS with del(5q). Their findings show a clear distinction between patients with 5q- syndrome and other MDS patients with del(5q). Unlike these latter, 5q- syndrome patients had no additional copy number changes, a finding that may indicate relatively genetic stability. These observations further support the definition of a separate entity of MDS with isolated 5q deletion that has been proposed both by Boultwood and Wainscoat5 and the WHO classification.9
Since isolated del(5q) is associated with good prognosis in primary MDS, proper recognition of this chromosomal abnormality is of fundamental importance. In this issue of the journal, Mallo et al.12 report findings of a study showing that fluorescence in situ hybridization (FISH) improves the detection of deletion 5q31-q32 in patients with MDS without cytogenetic evidence of del(5q). They correctly conclude that FISH of 5q31 should be performed in cases of a suspected 5q-syndrome in which the cytogenetic study has shown no metaphases.
The 5q- syndrome as a clonal stem cell disorder
The molecular basis for the 5q- syndrome has been the subject of extensive investigation for decades,5 but major advances have been made only recently. There is no question that the 5q- syndrome is a clonal disorder.13 It is, however, unclear how a clonal proliferation of hematopoietic stem cells can occur in a MDS. In myeloproliferative disorders, a somatic gain-of-function mutation of JAK2 provides hematopoietic cells with less propensity to apoptosis and a growth advantage determining clonal proliferation (Figure 2A).14 By contrast, in paroxysmal nocturnal hemoglobinuria (PNH) the clonal expansion of the PNH clone depends on the existence of one or more additional external environmental factors that damage normal hematopoietic stem cells and spare the PNH cells, thus exerting a selective pressure in favor of these latter (Figure 2B).15–17 Since myelodysplastic clones are defective with respect to both differentiation and maturation, it is unlikely that clonal myelodysplastic stem cells can have a growth advantage over normal hematopoietic stem cells. Thus, the most likely model for clonal proliferation of myelodysplastic stem cells is that of conditional selection:17 as in PNH, this dual pathogenesis would involve both the existence of stem cells with a somatic mutation and a failure of normal bone marrow. This latter may reasonably involve autoimmune mechanisms.
Haploinsufficiency of genes mapping to chromosome 5q31-q32 and aberrant ribosome biogenesis
The mechanisms responsible for failure of normal bone marrow in MDS patients are currently unknown. By contrast, recent observations indicate that haploin-sufficiency for one or more of the genes mapping to the common deleted region at 5q31-q32 (a dosage effect resulting from the loss of a single allele of a gene) is likely the pathophysiological basis of the 5q- syndrome.18 Candidate genes showing haploinsufficiency included the tumor suppressor gene SPARC and RPS14, this latter encoding a component of the 40S ribosomal subunit. Germline mutations in other genes controlling ribosome biogenesis – RPS19 and RPS24 – have been found in patients with Diamond-Blackfan anemia, a congenital disorder characterized by erythroid hypoplasia.19,20 In a very elegant study, Ebert et al.21 recently found that partial loss of function of RPS14 phenocopies the 5q- syndrome in normal hematopoietic progenitor cells, and that forced expression of RPS14 rescues the disease phenotype in bone marrow cells from patients with 5q- syndrome. Their observations suggest that defective erythropoiesis in the 5q- syndrome is caused by a defect in ribosomal protein function. In another recent study, Pellagatti et al.22 indeed found that patients with the 5q- syndrome have defective expression of genes involved in ribosome biogenesis and in the control of translation, suggesting that the 5q- syndrome represents a disorder of aberrant ribosome biogenesis. This abnormality cannot, however, explain the growth advantage of 5q- hematopoietic cells. Haploinsufficiency of the SPARC gene, encoding a protein with antiadhesive properties,23 might result in increased adhesiveness of 5q- cells to their bone marrow niche, but experimental evidence supporting this hypothesis is lacking. We, therefore, believe that a dual pathogenesis model likely operates also in patients with 5q- syndrome (Figure 2B). Were this to be true, 5q- cells would rescue the patient from bone marrow aplasia as cells carrying a mutant PIG-A do in PNH.
Lenalidomide treatment of myelodysplastic syndrome with del(5q): benefits and risks
Patients with the typical 5q- syndrome have a relatively good prognosis with a low risk of leukemic evolution. However, their anemia tends to worsen with time. Many of these patients have elevated serum erythropoietin levels, as do other patients with erythroid hypoplasia and a reduced rate of erythropoietin utilization,24 and do not, therefore, respond to recombinant human erythropoietin.25 Thus, until recently regular red cell transfusions and iron chelation26 represented the standard treatment for severely anemic patients with 5q- syndrome.
In December 2005 the US Food and Drug Administration (FDA) approved the use of lenalidomide “for the treatment of patients with transfusion-dependent anemia due to low- or intermediate-1-risk myelodysplastic syndromes associated with a deletion 5q cytogenetic abnormality with or without additional cytogenetic abnormalities”. Studies by List et al.27,28 had shown that lenalidomide is indeed able to induce a cytogenetic remission and to abolish transfusion requirement in a substantial portion of patients with MDS and del(5q). The drug was not developed for the treatment of this condition, and its mechanism of action is unclear. Nonetheless, lenalidomide inhibits growth of del(5q) erythroid progenitors in vitro,23 and likely inhibits del(5q) hematopoietic cells in vivo, at least in those patients who achieve cytogenetic remissions.28 It still remains to be established why treatment results in a quick recovery of red cell production while it is associated with long-lasting neutropenia and thrombocytopenia in many cases. We proposed that these divergent effects may be consistent with an anti-cytokine activity of the drug, which would favor erythropoiesis while inhibiting granulocytopoiesis and megakaryocytopoiesis.29
In a commentary29 to the first study on the use of lenalidomide in patients with MDS,27 we concluded that, although this treatment was promising, its feasibility and adverse effects needed to be defined more precisely in prospective studies. An European, multi-center, randomized, double-blind, placebo-controlled, three-arm study is currently evaluating the efficacy and safety of two doses of lenalidomide versus placebo in transfusion-dependent subjects with low- or intermediate-1-risk MDS associated with del(5q) (http://www.clinicaltrials.gov/ct/show/NCT00179621). The outcome of one of the patients enrolled in this study is reported in Figure 3 to illustrate the remarkable efficacy of lenalidomide in some patients with 5q- syndrome. It should be noted, however, that lenalidomide treatment of patients with MDS and del(5q) is challenging for clinicians and requires considerable hematologic know-how, especially because it may be associated with long-lasting grade 3–4 neutropenia and/or thrombocytopenia.28
Lenalidomide was designated as an orphan medicinal product in MDS by the European Medicines Agency (EMEA) on March 8, 2004. On January 24, 2008, the EMEA Committee for Medicinal Products for Human Use (CHMP) adopted a negative opinion, recommending the refusal of marketing authorization for lenalidomide, intended for the treatment of anemia due to MDS, more specifically for treatment of transfusion-dependent patients with MDS associated with del(5q) and with a low to intermediate risk of progressing to leukemia or death.30 Following the applicant’s request for a re-examination of the opinion, the CHMP confirmed the refusal of the marketing authorization on May 30, 2008. The CHMP concluded that the safety of lenalidomide was difficult to assess, and that, in particular, it was difficult to determine whether treatment with this drug increased the risk of progression to acute myeloid leukemia. In conclusion, the CHMP was of the opinion that the benefits of lenalidomide in the treatment of anemia of MDS with del(5q) did not outweigh its potential risks.
Since lenalidomide in unlikely to be mutagenic, a potential mechanism determining leukemic evolution might be selective pressure. Considering the model reported in Figure 2B and assuming that lenalidomide suppresses the 5q- clone (as in vitro23 and in vivo28 observations suggest) and allows restoration of normal hematopoiesis, a prerequisite for response is that normal residual hematopoietic cells are present in the patient’s bone marrow. The absence of a sufficient number of such stem cells would involve development of marrow aplasia with severe pancytopenia following the suppression of the 5q- clone that sustained blood cell production. Moreover, should a more abnormal subclone pre-exist and be unresponsive to lenalidomide, this subclone might emerge and lead to a more aggressive hematologic disorder.
As European hematologists who take care of patients with MDS we are experiencing a problematical situation. Almost none of the drugs used in the treatment of patients with MDS (including erythromulating agents) have an approved indication for these disorders in Europe. With few effective therapeutic options available, it is not easy for us to renounce a drug that can provide results such as those illustrated in Figure 3. On the other hand, the concerns of the EMEA CHMP are fully understandable, as our main duty as physicians is primum non nocere.
Celgene Corporation informed EMEA that it will continue to make lenalidomide available for patients included in clinical trials or compassionate use programs. The CHMP made the following statement for patients currently receiving lenalidomide: “If you are in a clinical trial or compassionate use program and need more information about your treatment, contact the doctor who is giving it to you”.30 As a doctor, I have no certainties, some hopes, and many doubts on this subject now, and find it extremely difficult to provide patients with accurate information about lenalidomide treatment. I believe it is imperative for us to identify those patients who could benefit from treatment with a low risk of adverse effects: transfusion-dependent patients with a typical 5q- syndrome5 likely fit into this category. By contrast, the efficacy of lenalidomide in MDS patients with del(5q) who have pancytopenia (neutropenia and thrombocytopenia in addition to anemia – a feature of patients with long-lasting disease), excess of blasts or additional chromosomal aberrations is questionable. More importantly, adverse effects are more likely to occur in these latter cases, mainly because few normal residual stem cells may be present. We hope that results of the ongoing multicenter, placebo-controlled study (NCT00179621) may help to clarify, at least in part, the current uncertainties on the use of lenalidomide in MDS patients with del(5q).
The author reported no potential conflicts of interest, in particular no financial relationship with pharmaceutical companies selling drugs employed in the treatment of myelodysplastic syndromes.
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